Fluid pressure braking system



O 1951' 'J. F. SHUMAKER FLUID PRESSURE BRAKING SYSTEM Filed April 14, 1950 Q m W J k @u M MW ATTORNEYS Patented Oct. 9, 1951 UNITED STATES PATENT OFFICE- John F. Shumaker, Elyria, Ohio, assignor to Ben- (fix-Westinghouse Automotive Air Brake Company, Elyria, Ohio, a corporation of Delaware Application April 14, 1950, Serial No. 155,842

15 Claims. (o1. 3oss4) This invention relates to fluid pressure braking systems for tractor-trailer vehicles and more par- ,ticularly to an arrangement for conserving fluid vided for the usual service applications of the vehicle brakes while the emergency lines are utilized for automatically applying the trailer brakes in the event of rupture or severe leakage in the emergency lines. During the usual service brake applications, fluid pressure is conducted from a suitable source on the tractor to the tractor brake chambers and in addition, such pressure is soconducted to the trailer service line in order to effect operation of a relay valve which thereby connects a trailer reservoir to the trailer brake chambers. Should the tractor or trailer service lines become broken or develop a severe leakage, fluid may be released from the source so rapidly that an effective service brake application on the tractor could not be obtained. In addition, and under such conditions, the leakage of fluid may be such as to prevent the trailer relay valve from functioning and thus a total failure of both the tractor and trailer brakes would result.

It .is accordingly'the principal object of the present invention to provide a fluid pressure brake system which is so constructed and arranged as to avoid the objections and disadvantages referred to above.

Another object is to provide in a tractor-trailer fluid pressure braking system, a novel construction for conserving the fluid pressure on the tractor in the event of a severe leak or breakage of the service brake lines of the tractor or trailer.

A further object is to provide a novel control device carried by the tractor which is so constituted that efficient braking of the tractor may be accomplished notwithstanding the fact that the service line on the trailer is open due to a rupture, or develops a severe leak.

A still further object is to provide a novel safety and control arrangement which is effective to interrupt the flow of fluid pressure from the trac- Y tor service line to the trailer service line in the event the pressure differential between two spaced-apart points in the tractor service lines exceeds a predetermined value.

Still another object resides in the provision of a cut-ofi valve positioned in the tractor service line which is operative to positively close and prevent substantial loss of fluid pressure from the tractor braking system when a severe leak occurs in the trailer braking system, such an arrangement thereby enabling the tractor brakes to be efliciently applied.

Still another object comprehends the provision of a relatively simple control device of the above character which is positive in its operation, and which does not interfere in any manner with the normal functioning of the brake systems on the two vehicles.

Other objects and novel feature of the invention will appear more fully hereinafter from a consideration of the following detailed description when taken in connection with the accompanying drawing illustrative of one form of the invention. It is to be expressly understood however, that the drawing is utilized for purposes of illustration only and is not to be taken as a definition of the limits of the invention, reference being had for this latter purpose to the appended claims.

Referring to the drawing, the single. figure thereof diagrammatically illustrates the present invention in connection with a tractor-trailer braking system of the fluid pressure operated type, certain of the parts being shown in section.

More particularly, and as shown in the drawing, the present invention is employed in connection with a. tractor-trailer braking system of a well known type adapted to be operated by air under pressure. For example, the system includes a compressor ID forsupplying compressed air through a conduit l2 to a pair of serially connected reservoirs l4 and Hi, the loading and unloading of the compressor being preferably controlled by governor l8 connected to the compressor [0 by -means of a conduit 20. As shown, reservoir 16 is a protected reservoir, in viewvof the incorporation of a check or other suitable type of valve IS in the connection I'L Governor I8 may be of any suitable construction such asthat disclosed, for example, in the patent to B. S. Aikman No. 1,754,218, dated April 15, 1930. Front and rear braking chambers 22 and 24 are provided for respectively applying the front and rear brakes to the tractor and these chambers may be supplied with air pressure from the reservoir I4 through conduits 26, 2.1, 28, 30, 32, 34, 3B and 38, a suitable pedal operated brake valve 40 being associated with the conduits 26, 21 and 34 in order to control the degree of brake application of the front and rear brake chambers. The brake valve 40 may be of any suitable construction well known in the art, an example being shown in the patent to W. J. Andres et al. No. 2,133,275, dated October 18, 1938.

In order to provide suitable connections for the trailer brakingnsystem, the, tractor braking system includes an emergency line :42 as fwell as the service line 44. The emergency and service lines 42 and 44 terminate in coupling parts of, well known construction which are adapted to be coupled with similar parts;associatedwith'.

the trailer braking system in order to provide releasable couplings 46 and 48. Iniaddition," the trailer emergency and service lines arelrespec-e tively provided with cut-off cocks 50 and 52' in order to positively cut off theselines'whenther couplings 46 and 48 are disconnected. prior, to

separation of the vehicles.-

The trailer braking system includes a plurality of .brake chambers, two-of which are illustrated at -54and" 56. Such system also-includes an emergency line 58;aserviceline 60 and'a trailer reservoir- 62, all ofthe foregoing being inter-" connected through a" relay-emergency valve 54 of "any suitable construction; such as for example that shown'in the patent 1706. Johnson, J r:, No; 2,818,212, dated October 22, 1935. Ingthe'normal operation of the valve e4, the" emergency-dine '8" is-normallyponnected with' the reservoir 52 iniiorder'to maintain the latter charged with air pressure fromthetractor'reservoir IS. The valve 64"isalso'of'suchconstruction as to connectthe trailer reservoir 62 with the brake chamb'ers'54 and 56 through conduits Eli-and (it whenever the serviceili'nefill is'charged with fluid pressure by reason of the: operation" of the" tractor: brake valve'40. The construction is moreover'a-lso such, thatiin theeventofa rupture orisevere leakage of the'emergency line'58, the-valve 64 functions to automaticallytsupply air pressure to the'brake chambers 54 and'8 from. the reservoir; 62' in order to effect an emergency application of ":the trailer-brakes.

From the foregoingdescription, it will heap preciatedthat in order to effect a normal service application of the tractor. and trailerbrakes; it is only necessary fol-the operator'to actuate:the pedal'of the brake valve4il. This'actionserves to conduct air pressure from the reservoir I41to the tractor front b'rake" chambers 22 'as by way ofjconduits 26, 21, 28, 3Iland32. Simultaneous= ly, air-pressure is .also,conducted to'the tractor rear" brake chambers 24 'through' conduits" 34,

3B and38. addition; the tractor-'serviceline' 44 "is charged'withfiuid pressure through" a conduit II! in order to supply 'air pressure' to" the trailerservice line .60. to effect an application of the trailer brake chambers 54Vand 56through operation of the. valve. 64 which controlsothe' trailerreseryoir-fiil;

In the event that a severe leakage occurs in the service linesofthetractor or trailer, it will be readily understood that uponactuation of the brakervalve'll, the fluid pressure in the reservoir I4- might be depleted so rapidly that efli= cient applicationxof the tractorrand trailer brakes could notube-effected. Sucha severe leak, might be caused byia: broken servicelinezon the tractor or :trailer or .by reasonuofg'an accidental parting ofthe couplingse46 and48. By the present. ine vention, this difficultyis overcome andlan efficient "application of the tractor brakes may be efiected notwithstanding that the-service -lines 4 have developed the severe leakage just referred to.

More particularly, the present invention includes a novel arrangement for positively interrupting the flow of air through the tractor service line 44 in the event that a predetermined pressure differential between two-spaced-apart points exists. Such a, pressureedifierential would be caused by a broken or'ruptured service line and in such event, the construction provided by the present invention causes the flow of air pressure-:through" thestractor service line to be interrupted. More particularly, the invention includes'a' fluidlpressure responsive cut-off valve I2 which is "positioned on the tractor and is arranged "in the tractor service line 44. As shown, thezz-valvew'll.includes a casing I4 having inlet andloutletports I6 and I8 which are respectively connected'with the conduit 10 and the tractor service line 44. Normally, the ports I6 and I8 communicate witheach other past a-normally open valve which is carried by 'apressure -re= sponsive element 82. In the'position shown;- the elementti is moved downwardly in responseto the action of 'a spring 84. application of fluid pressure beneath theelement 82, the elemen-tand the-valve- '80 will bemoved upwardly in order'to bring thelatter into' contact with a seat 86" and i thus interrupt flowof fluid pressure from the-inlet 'port '76 to the cut let'port 7-8;

A novel'arrangement is-provided bythe present invention for controlling--'the applicati0n-of 'fiuid pressure to '-the-'- cut-off valv'e' I2, and more par-i ticularly, such arrarrangement' is so constituted as to cause'operation of the valve to close'ofi the service line-44 iirthe-event that theservice-lines of the vehicles are broken" or' develop serious 1 leakage which mightbe' of such magnitude as to prevent effective application of any of the tractor brakes; More particularly, the' foregoing con-' struction includesa control valve device '88 which includes a pi1otva1ve-=-9Il adapted to be operated and controll'ed by movement of a pressure re-' sponsive element orpiston' 92,- and also'includes the reservoir I6 whichprovides a' protected source of fiui'd pressure for controlling the cutofi valve-I2.-

For the purpose'of effectively controllingthe pilot valve 90 and'also the "cut oif valve", the

action of the piston 92 is controlled by'an' air' pressure difierential occurring between two spaced-apart-points inth'e tractor service line, illustrated atA and B in the drawing. With'such 1 an arrangement, fluid pressure :fromthe conduit Iii-may be conducted "to' a pressure chamber '94 above the piston 92 through a conduit96. Also; fluid pressure from point 'B may 'b'e'conductedto a" control chamber 98 beneath'the piston 92' by Way-"of a condui-t I00; It will be observedthat the connection betweenconduit-Ifll) and control chamber-'98 incluudesa normally spring-closed check-valve I 02 and a by-pass duct Ill4-controlled by aneedlevalve lfifltheseparts being provided fora purpose to bepointed out more fully here'- inafter;

In additionto the piston 92, the valve device 88 includes a diaphragm I08 which is clamped"between upper-and lower casing sections H0 and I I 2. The chamber] I4 beneath the diaphragm is normally open to atmosphere through an'exhaust port I lfi and such chamber 'also includes -a'spring I I8 for resiliently opposing downward movement of thediaphragm I08. A valveactuatin'g member I22, carried by-the diaphragm I08-extends However, upon* the into an outlet chamber I24, and as shown, the member I22 is normally spaced from an exhaust valve I26 in order to connect the chamber I 24 with the atmosphere by way of bore I28, openings I30 and port H6. With such an arrangement, it will be readily understood that in the normal position of the parts illustrated, chamber I32 beneath the element 82, and communicating with the chamber I24 through conduit I33, will be subjected to atmospheric pressure and the cutofi valve 12 will remain open. However, under certain conditions, as will appear hereinafter, the piston 92 will be moved downwardly so that a piston rod I34 carried thereby and spaced from the member I22, as shown, will engage and move the member I22 to a valve opening positioned. In such event the chamber I32 will be charged with pressure from the protected reservoir I6 by way of conduit I38, inlet chamber I40, open inlet valve I42, chamber I 24 and conduit I33 in order to close the valve 80. Thus when the piston 92 is moved downwardly to open the pilot valve 90', the service line 44 on the tractor will be closed through operation of the cut-off valve 12.

Preferably the invention provides an arran gement whereby the cut-off valve I2 remains closed except when manually released. This function is achieved by connecting the outlet chamber I24 to chamber I44 above the diaphragm I08 by way of conduits I46 and I41 together with a manually operable three-way valve I48 therein. Thus when the valve I48 occupies the position shown in full lines, reservoir pressure will 'be conducted through the valve to the chamber I44, and the diaphragm I08 will maintain the pilot valve 90 open notwithstanding the return of the piston 92 to the normal position illustrated. Hence it will be appreciated that the cut-ofi valve I2 will remain closed. If it is desired to open the cut-off valve after the piston 92 has returned to its normal position, it is only necessary to move the valve I48 to its dotted line position to block the conduit I46 and connect the chamber I44 to an atmospheric port I50.

In normal operation, brake-applying movement of the tractor brake valve 40 serves to charge the tractor brake chambers 22 and 24 and to also charge the conduit I0 and the tractor and trailer service lines 44 and 60. As well understood, the tractor brakes will then be applied and the trailer brake chambers 54 and 56 will become charged from the trailer reservoir 62 through operation of the relay-emergency valve 64. During such charging of the conduit I0, the piston 92 of the control valve device 88 will not move a sufiicient distance to open the pilot valve 90. This action will be readily understood from the fact that the pressures at points A and B are substantially equal and that the fluid from point B will readily flow past the check valve I02 to the control chamber 98. Hence the pressures above and below the piston 92 will substantially immediately become balanced. While initial charging of the conduit 19 may produce a slight downward movement of the piston 92, this slight movement is permitted bythe spacing of the piston rod I 34 from the member I22. Moreover such movement is resisted by a return spring I52 and the dash pot action provided by the atmospheric air within the chamber 98 which is partially trapped therein by the restriction afiorded by the needle valve I06. Hence under these conditions, any movement of the piston 92 will not be sufii'cient to move the member I22. During release of the tractor and trailerbrakes, the conduit I0 and the service lines 44 and 60 will be exhausted and the pressure chamber 94 will be likewise exhausted through conduits 96 and I0. Due to the restriction of the "needle valve I06, control chamber 98 will be exhausted at a slower rate than chamber 94, but will be substantially promptly reduced to atmospheric pressure by way of the needle valve I06, duct I04 and conduits I00 and I0. Thus during ordinary service brake operations, the control valve device will not operate and the cut-ofi valve I2 will remain in open position through the action of the spring 84.

On the other hand, and assuming that the service line connection between valves 12 and 64 is ruptured or develops a severe leak, then in that event, the present invention is effective to close the valve I2 upon operation of the brake .valve 40 and this action conserves the supply of air on the tractor and enables an efiective application of the tractor brakes to be accomplished. More particularly and under the above mentioned conditions, when the tractor brake valve 40 is applied, the pressure chamber 94 will be charged with fluid pressure from point A through conduit 96 and the downward movement of the piston 92 will be initially retarded by the atmospheric air trapped in the chamber 98 by the check valve I02 and the restriction of the needle valve I06. While fluid pressure from point B may subsequentlybe admitted to the control chamber 98 past the check valve I02, it will be understood that the differential of fluid pressure between points A and B, due to the leakage in the service lines 44 or 60 is substantial. Hence, after an interval of time determined by the setting of the needle valve I06, or other variable equivalent restriction, the fluid pressure differential existing on opposite sides of the piston 92 will be suflicient to effect downward movement thereof in order to move the member I22 and operate the pilot valve 90. During such movement of the member I22, the exhaust valve I26 will be first contacted in order to close the exhaust connection to the chamber I32. Thereafter, continued movement of the piston rod I34 downwardly eifects an opening of the valve I42 in order that the chamber I32 may be charged with fluid pressure from the protected reservoir I6 by way of conduit I38, chambers I40. and I24 and conduit I33. Thus the cut-off valve 12 will close and'prevent any further loss of fluid pressure from the tractor service line 44 and the tractor brakes may hence be effectively applied.

From the foregoing description, it will also be understood that upon operation of the pilot valve 90, the chamber I44 will also be supplied with fluid pressure from the protected reservoir I6 by way of conduit I38, chamber I40, conduit I46, the open valve I48 and conduit I47. This action serves to keep the pilot valve open to maintain the cut-01f valve I2 closed notwithstanding the movement of the tractor brake valve 40 to its exhaust position. Hence even though the trailer service line 60 develops a severe leakage, repeated applications of the tractor brakes may be made in orderto safely control the vehicle train pending repair of the break.

In the event the leakage referred to is due to a break-in-two of the vehicles, it will be understood of course, that parting of the emergency lines 42 and'58 will automatically apply the trailer brakes through operation of the relay-emergency valve 64 and that the cut-ofi valve 12 will close as described above. Although theztractori emergency 1 line: 42 willi be 1 vented to atmosphere;- under these conditions, the; loss 2 in pressure fromthe tractor: reservoir; 1 4 will, not be suificient'. to prevent a an; application: of the tractor? brakes; Thus. during operation of the tractor; brake valve 40 following 1' a' parting; of the vehicles, the parts including-the control de-= vice--B8,- theipilot valve 90 and the cut-oif valve 12 will function; in the manner previously set forthziniorder to-"cutoff the flow'of fluid pressure-through the tractor service line-44. Hence even though pressure is being exhausted to :at mosphere through: the open tractor emergency line 42, still suflicient-pressureywill ber available. for effecting an efl'ective braking 1 of: thetractor.

From the foregoing; it will bewreadily per. ceived that the present L invention; provides-:.. a novel .and; efficient braking system "wherein the compressedair: on the tractor is conserved:;fr tractor braking purposes in thei-event of-iawzbroken'service lineor apartingpf the vehicles: The incorporation of thecut-oirvalve :lziinsures'zthat the --lossof asubstantial amount of available air, pressure-fromuthe tractor reservoir. will be prevented, ,even though the serviceandvemergency-linesof thevehicles mayboth be open to atmosphere. It will .also be understood: that while the control device 8Bz'is'sozarrangedas to beazinoperative during brake applications ,under. normal conditions, it ,nevertheless iseffective to CIOSCyth'GJCllt-Ofi valve"12 in-;case :auserious leakage in the tractor" or trailer; service; lines occurs; In; such event, the control device- 88 is moved 'inaccordance with. the pressure differential exi'sting'ga-t spaced-apart: points in the tractor "serviceline-after a- -time delay interval determined by the setting of the needle valve l 06 Eor other .restriction' in :parallelwith the check valve- 102; Under normal conditions, the: use off the check valve l02':and-;the-;restricted bye pass I04 provides-a: dash-pot action for; the

pistonn-SZTin order to prevent. movementof: the

latter during initialzcharging of theachamber 94-, and until such time as the :fluidirom point B-rflows tothecontrol chamber 98::past-the check valve and balances theepressure in 'theepressure chamber '94;

While; one embodiment 1 of the invention has been showncand described herein-:with considerableparticularity'; itr'will'be readily understood by "'thOSG' SKiHedfTiII tithe art that various modifl-t cations may be restorted: to: without departing from' the spirit of the-invention; Rerefenoe will therefore be had to the appended claimsifori'a definition of therlimits: of 'theiinventionii What is claimed-1st? 1; In afluid pressure operatedfvehiclei braking system; a source *of fluid pressure; a plurality of brake chambers, a brake valveconnectedwvith said source, aplurality of conduitsconnecting the brake valve -'.-and chambers, a: normally? open fluid'tpressure actuateda'valve'in one vof th'econ duits, and-means to supply fiUidlpIESSUIB?1303131618 latter 'valve. to close.- the :same. and? thereby inter= rupt the;v flow of fluid: pressure therethi'oug-h', comprising: at fluidapressure: 'difierenti'al operated control device including afiuid pressure actuator; means to subject ,OH'GSidETOf :said actuator :to :the pressureexistingat one point in said one: conduit, means to subjectith'e opposite side-:of -said-iactue ator to the pressure'existing ,at another rpointain said one conduit, ax checkivalveassociated with the last named-means,- andz'a restrictedafbyj pass around the-check valve.

2.. .In a fluid, pressure: operatedyehicle ibraking system; a; source of fluid pressure aeplurality of brake, chambers, a brake valve connected vawith said source, a plurality of ,conduitsconneoting the brake valverandi chambers, anormally; open fluid pressure-actuated valve :in' one of the conduits, and "means to supply fluidlpressure v.to the latter valveatov close th same. and; thereby interrupt the flow of fluid pressure therethrough, comprising ar-pilot valve, a fluid actuator including a .prese surev responsive. element movablefor operating said, pilot valve, a .conduitfor, subjecting one side of said,element .to thepressure existingat onepoint a in said one conduit, ,a-secondconduit for: subjec-tingathe opposite side of the element to: the .ipressurerexisting at another point in. said one conduit, ,a checkvalve in said second conduit, and agrestrictedby-pass around the" checklvalve.

3 In a zfluid, pressure operated vehicle braking system,a:source:of fluid pressure, a plurality of brake chambers: a brake, valve connected 7 with saidsource; a plurality of conduits connecting the brake valve andchambers, a normally openfluid pressurev actuated valve:-in one of the conduits, and means to supplyfluid pressure to -thelatter valve to, close they same :and thereby interrupt the now of ,fl-uidpressure .therethrough comprispressure ,responsiveelement movable foroperating saidlpilot valve, .saidactuator being provided with vaupressure. chamber on one sideloflsaid element and .a .control chamberfionith'e opposite side of .saidelment, means afiording anunrestricted connection .s between the pressure chamber andsaid, one conduit at a predetermined point therein, means. connecting .the control chamber to said one conduit-at another pointspaced. from the firstrnamed .pointso thatisaid. element will be...subjected to a fluid pressuredifferential occurring betweensaid, points euponi flow of fluid pressurethrough saidonecond-uit, and means associatedwith said control chamber for prevent: valve operatinggmovement of said. element when. said pressure differential is. less vthan a selected value and for allowing valve. operating movement .of said element whensaid pressure difierential reaches or exceeds said selected .value.

4. In a fluid pressure operated vehicle braking system a source of fluid pressure, a plurality of brakechambers, abrake Valve. connected with said source, a plurality ofconduits-connecting the brakevalve and chambers, a normally? open fluid pressure actuated valve inone of the cond-uits, and means tosupply fluid pressureto the latterivalvento closethe same and therebyinterrupt theqflow of fluid pressure therethrough come prising ,apilot valve, a fluid actuator :including a. pressure responsive -e1ement movab1e for. operating saidpilot valve, said actuator being provided-with-a pressure chamber-onone side of said element .and a? control chamber on the" opposite side of said element, means aifording an'unrestricted connection between the pressure chamberand said one conduit at a predetermined point therein, meansconnecting the-control chamber to said oneconduit at another point spaced from the first named point so that said element will be subjected to aIfluid pressure differential occurring between. said points uponflow of fluid pressure through said one conduit, and means associated withzsaid: controlchamber for 1' preventing valve operating movement of'saidelement when said pressure differential is less than a selected: value and for allowing valve operating movement of said element when said pressure differential reachesor exceeds said selected value; saidilast '9 named means including a check valve in said connecting means arranged to allow flow of fluid to the control chamber only, and a controllable by-pass around the check valve.

5. In a fluid pressure operated vehicle braking system, a source of fluid pressure, a plurality of brake chambers, a brake valve connected with said source, a plurality ofconduits connecting the brake valve and chambers, said brake valve being operable to connect said source with said conduits to charge the latter and to connect the conduits to atmosphere to exhaust the same, a normally open fluid pressure operated valve in one of the conduits, means including a pilot'valve movable to a position for connecting said source to said fluid pressure operated valve to close the latter when the pressure differential created between spaced-apartpoints in said one conduit due to the flow of fluid therein reaches a predetermined value, and means for maintaining said fluid pressure operated valve closed after said pressure differential is reduced to zero due to the exhausting of said one conduit.

6. The combination as set forth in claim which comprises in addition, manually operable means for releasing said maintaining means.

7. In a fluid pressure operated vehicle braking system, a source of fluid pressure, a plurality of brake chambers, a brake valve connected with said source, a plurality of conduits connecting the brake valve and chambers, said brake valve being operable to connect said source with said conduits to charge the latter and to connect the conduits to atmosphere to exhaust the same, a normally open fluid pressure operated valve in one of the conduits, means including a pilot valve movable to a position for connecting said source to said fluid pressure operated valve to close the latter when the pressure differential created between spaced-apart points in said one conduit due to the flow of fluid therein reaches a predetermined value, and means for maintaining said pilot valve in its connecting position after said pressure differential is reduced to zero due to the exhausting of said one conduit.

8. The combination as setiorth in claim 7 which comprises in addition, manually operable means for releasing said maintaining means.

9. In a fluid pressure operated vehicle braking system, a source of fluid pressure, a plurality of brake chambers, a brake valve connected with said source, a plurality of conduits connecting the brake valve and chambers, said brake valve being operable to connect said source with said conduits to charge the latter and to connect the conduits to atmosphere to exhaust the same, a normally open fluid pressure operated valve in one of the conduits, means including a pilot valve movable to a position for connecting said source to said fluid pressure operated valve to close the latter when the pressure differential created between spaced-apart points in said one conduit due to the flow of fluid therein reaches a predetermined value, and means for maintaining said pilot valve in its connecting position after said pressure differential is reduced to zero due to the exhausting of said one conduit, said last named means including a fluid pressure actuator communicating with said source and said fluid pressure operated valve when said pilot valve is moved to said connecting position.

10. The combination as set forth in claim 9 which comprises in addition, manually operable means for releasing said maintaining means.

11. In a fluid pressure operated tractor-trailer W braking system having interconnected tractor and trailer service and emergency lines, a source of fluid pressure on the tractor, a tractor brake valve for controlling the flow of fluid pressure from said source through the service lines, and means for interrupting said flow through the tractor service line comprising a normally open fluid pressure responsive valve in the tractor service line, a fluid pressure actuator movable in response to a predetermined pressure differential created between spaced-apart points in said tractor service line, means including a pilot valve operable by movement of said actuator for supplying fluid pressure to the fluid pressure responsive valve to close the same and thereby interrupt said flow through the tractor service line, and means for maintaining said fluid presure responsive valve closed after the pressure in the tractor service line is reduced to zero.

12. The combination as set forth in claim 11 which comprises in addition, manually operable means for releasing said maintaining means.

13. In a fluid pressure operated tractor-trailer braking system having interconnected tractor and trailer service and emergency lines, a source of fluid pressure on the tractor, a tractor brake valve for controlling the flow of fluid pressure from said source through the service lines, a normally open fluid pressure actuated valve in the tractor service line, a pilot valve normally occupying a first position connecting the fluid pressure actuated valve with the atmosphere and movable to a second position to interrupt the atmospheric connection and connect said source with the fluid pressure actuated valve to close the latter, a fluid pressure operated device for moving the pilot valve to said second position, and means for subjecting said device to a pressure difierential created at spaced-apart points in said tractor service line.

14. In a fluid pressure operated tractor-trailer braking system having interconnected tractor and trailer service and emergency lines, a source of fluid pressure on the tractor, a tractor brake valve for controlling the flow of fluid pressure from said source through the service lines, a normally open fluid pressure actuated valve in the tractor service line, a pilot valve normally occupying a first position connecting the fluid pressure actuated valve with the atmosphere and movable to a second position to interrupt the atmospheric connection and connect said source with the fluid pressure actuated valve to close the latter, a fluid pressure operated device for moving the pilot valve to said second position, means for subjecting said device to a pressure differential created at spaced-apart points in said tractor service line, and means for maintaining said pressure actuated valve in closed position after the pressure in the tractor service line is reduced to zero.

15. The combination as set forth in claim 14 which comprises in addition, manually operable means for releasing said maintain n means.

J OI-IN F. SHUMAKER.

REFERENCES orrcn The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,797,431 Mathieson Mar. 24, 1931 1,969,812 McCune Aug. 14, 1934 2,084,707 Robinson June 22, 1937 2,407,890 McClure Sept. 17, 1946 2,451,139 Williams et al Oct. 12, 1948 

